[unreadable] [unreadable] In the United States, the occurrence of breast cancer (BC), the most common cancer in women, has increased for the past two decades. Prevention of this cancer is difficult because most identified factors contributing to breast cancer occurrence cannot easily be modified. Two factors, obesity and physical inactivity, are characteristics that have both been shown to be associated with a higher risk of BC occurrence and to have the potential to be changed. However, public health efforts to reduce the rapidly increasing rates of obesity and increase physical activity, stated goals of the NIH, have yielded limited success. Therefore, new directions and strategies are needed to combat the increasing occurrence of BC. Studies are now providing convincing evidence that physical activity and obesity are associated with the built environment, man-made attributes of a person's surroundings such as the existence and condition of sidewalks, availability of resources (health care, food, playgrounds, public transportation) and the number of walkable destinations. However, no prior studies have considered the influence of the built environment on cancer incidence. Combining survey data collected from the California Teachers Study, a prospective cohort of over 120,000 women, with clinical breast cancer information from the California Cancer Registry, and built environment characteristics from geographic information systems (GIS), this will be the first study to test the hypothesis that the built environment affects the risk of BC. In doing so, this study will 1) determine the specific measures of the built environment that are related to physical activity and obesity; 2) test whether these built environment measures are related to the risk of BC; and 3) determine if other factors related to BC occurrence, including menopausal status and socioeconomic status, change these associations. [unreadable] [unreadable] [unreadable]